We propose to characterize the morphological and functional development of an identified vertebrate neuron that occurs during its differentiation in vivo. The work will be carried out on the Mauthner neuron (M-cell) in embryos of the zebra fish Brachydanio rerio (40 - 150 hr following fertilization). Studies are proposed to quantitatively delineate the pattern of M-dendritic growth that occurs during this period and the appearance of morphologically identifiable synapses (light and electron microscopy). The origin of some of these synapses would be documented with anatomical tracing methods (horseradish peroxidase). We propose, in addition, to describe the development of functional sensory pathways which will bring the M-cell to threshold (electrophysiology).